1. Field of the Invention
This invention relates to that disclosed and claimed in U.S. patent application, Ser. No. 08/867751, filed Jun. 3, 1997, now U.S. Pat. No. 6,018,643. This invention relates to the use of superresolution techniques for the purpose of identifying a received signal from a class of signals which could possibly be received and, in the preferred embodiment, to the use of such techniques in an Advanced Mobile Phone System ("AMPS").
2. Background of the Invention
The AMPS is the most common cellular telephone standard in the United States. Each phone conversation uses two 30 kHZ bandwidth frequencies, one for the uplink (mobile phone to the base station) and one for the downlink (base station to the mobile phone). Since there are a limited number of frequencies available, it is common to reuse frequencies throughout the network. In other words, several base stations which are assigned the same frequencies are separated by some distance to keep the interference at tolerable levels.
In spite of the spatial separation, it is still possible for a mobile telephone or base station to detect the wrong signal (conversation). To help minimize this possibility, the AMPS system incorporates Supervisory Audio Tones ("SATs") in each transmission. There are three distinct SATs. These are at frequencies of 5970 Hz, 6000 Hz, and 6030 Hz. Each base station is assigned one of the SATS, and transmits this tone along with the audio conversation. The mobile telephone detects the received signal, filters the SAT from the audio, notes which tone was received, and transmits the same tone back to the base station on the uplink frequency. Similarly, the base station detects the received signal, filters the tone from the detected audio, and notes which tone was received. Should either the mobile telephone or the base station determine that an incorrect SAT was received (indicating a transmission from an interfering user), the audio is gated off so that the users do not hear the incorrect conversation. Should this situation (detection of the incorrect SAT) persist, corrective action such as reassignment of the phone call to a different channel or ultimately dropping the call can be performed.
FIG. 1 illustrates a prior art apparatus which can be used to determine which of the possible SAT tones has been received with a given FM signal. The FM input signal 2 is demodulated in demodulator 4 and is then coupled to the inputs of three parallel bandpass filters 6, 8, and 10, each centered at one of the SAT frequencies, and each having a bandwidth of 30 Hz. The delay time of the signal through the filter is roughly the inverse of the bandwidth, that is about 33 msec in the present case. Because of the presence of surrounding cells, the received FM input signal may contain energy at any or all of the SAT frequencies. The filter outputs represent the amount of such energy at each of the three frequencies. The amplitudes of SAT signals at each of the SAT frequencies are then estimated by a short averaging process in amplitude estimators 12, 16, and 18. SAT amplitude comparison logic circuit 20 then determines whether or not the signal having the largest SAT signal is at the correct frequency. While the 33 msec filter delay time is acceptable in some applications, that time is unacceptably long in others. Such is the case where the SAT identification is to be used in connection with adaptively forming the beam pattern of the antenna used at the base station of the system.